Tumors arising from terminal bronchiolar epithelium (Clara cells) will be investigated as a model of human bronchiolo-alveolar carcinoma. Cell kinetics and growth of both alveolar type II adenomas and Clara cell adenomas will be compared in mice exposed transplacentally to ethyl-nitrosourea; and the possible progression from the alveolar to the papillary tumor investigated. The preneoplastic period of bronchiolar cell hyperplasia in mice exposed to urethane will be studied in regard to specific enzyme function of the Clara cell.